The present invention relates generally to the preparation of printed circuit boards for mounting surface mount components and, more particularly to a method and apparatus for mounting surface mount components on printed circuit boards utilizing solder bumps.
The greatly increased use of very large scale integrated circuits, chip resistors, chip capacitors and redesigned inductors and connectors is making surface mounting rather than through-hole mounting of components the technology of choice for printed circuit board (PCB) assembly. Higher packing density of surface mount components (SMC) reduces solder joint size and spacing between components and greatly increases the number of solder joints on each PCB making board inspection more difficult and perfection in soldering imperative. Conventional through-hole component insertion and wave soldering processes are not suitable for the task of surface mounting of components. Further, surface mounting of components places a greater emphasis on the quality of starting materials and process control than is required with conventional wave soldered through-hole components.
Chip carrier or other components are typically surface mounted by soldering component contact pads or leads to corresponding contact pads on a supporting substrate which may be a PCB or the like. Solder must be supplied to the soldering sites in a quality sufficient to wet the contact leads and close all gaps between the component leads and the PCB pads.
It is well-known to screen print solder paste using silk screens or metal foil stencils to apply uniformly thick deposits of solder at predetermined, discrete locations over relatively large circuits such as are found on PCBs. An alternative method of depositing solder paste is multipoint dispensing. In both cases, component leads are immersed in the wet solder paste deposit after which the solder paste is oven cured and then reflowed. A major problem with solder paste is that of solder spatter resulting in small droplets or balls of solder ejected from the paste by gases being evolved from within the paste during rapid solder reflow.
A well-known method which avoids the solder spatter problem is to attach small spherical solder preforms referred to as "solder bumps" to the contact pad areas of the PCB. U.S. Pat. No. 4,558,812 entitled "Method and Apparatus for Batch Solder Bumping of Chip Carriers", issued to Alred S. Bailey et al on Dec. 17, 1985 discloses a technique which makes use of a vacuum plate having a plurality of arrays of dimples, each of the dimples containing a solder preform, to deposit a desired array of solder bumps on a PCB. Another method of forming solder bumps comprises stencil or screen printed or dispensed deposition of solder paste in a desired pattern on a PCB followed by both the oven cure and reflow steps to provide rounded solder bumps prior to placement of the components to be surface mounted. Once the solder bumps are formed, the PCB is cleaned aggressively with solvents and brushing as required to remove any solder spatter deposits. A small quantity of adhesive solder flux is applied to the peaks of the solder bumps by stencil printing or other suitable means before the alignment and placement of the components. The flux holds the SMCs in place during the solder reflow phase.
Although the above-described solder bump process effectively solves the solder spatter problem, maintaining accurate placement of the components is difficult and the reliability of the solder joints is reduced due to misplacement of the component leads. Further, the geometry of the solder bumps makes it difficult and time consuming to apply the flux.